Discrepancy with conversions tracked in Google/AdRoll vs. Marketo

Hello,

Working with our PPC company, we are seeing that the number of leads coming through each month (being captured in Marketo) vs. the number of conversions per month (leads being captured in Google & AdRoll) are not lining up.  The PPC platform numbers are almost always higher – almost significantly higher than what is being tracked in Marketo.  We have hidden PPC fields on all our forms and are tracking conversions based off these fields.  Has anyone else seen this? 

Thanks!